Automatic screw machines, CNC lathes and similar bar fed machines have been used for many years to machine small parts in large volumes. These machines provide a reliable, precise and economical way for producing a wide variety of parts, including air and hydraulic fittings, hose fittings, electrical fittings and connectors, carburetor parts, spark plugs, and various transmission and brake parts including shafts, pins, ABS brake cylinder pistons, screws, gears, splines, grease fittings and the like.
In many of these machines, a bar loader supplies bar stock to a plurality of work spindles mounted on a rotary spindle carrier. The carrier is indexed from work station to work station. Tools such as dovetail and circular form tools, drills, taps, reamers, thread rolls and shave tools, located at the different work stations, perform a variety of machining steps on the bar stock. When the machining sequence has been completed, the finished part is severed from the stock, and the stock is advanced so that the cycle may be repeated. When a length of stock has been consumed (referred to frequently as "stock depletion"), another bar of stock must be loaded into the work spindle. If stock depletion goes undetected, or if stock is not fed properly because the stock is undersize or the tension on feed fingers has dropped through wear: 1) The machine can run empty for some period of time--as the name "automatic" implies, once setup these machines run virtually unattended, and without stock depletion detection are often found running empty--causing lost productivity. 2) If there is only enough material to partially feed stock, tooling such as part pick offs will malfunction when the short part comes around. These situations can result in what can only be described as a "wreck"--which can mean destroyed tools, damaged machines, further lost productivity while the machine is retooled, or even cutting oil fires ignited by the friction from wrecked tools tangled with damaged stock. 3) Finished parts are generally deposited in a simple parts basket by the machine when completed. If short fed out stock successfully navigates its way through the tooling stations, a too short part will be deposited in the parts basket along with all the good parts. Many customers will reject a complete order if they find even one part in their shipment not made to the print. To avoid this possibility, many screw machine shops are doing 100% inspection in order to sort out the short pieces from the good ones. This is very expensive. Thus, it is vitally important to be able to detect stock depletion as soon as it occurs and take corrective action. At the same time, however, it is highly desirable to use the maximum amount of any length of stock before restocking, in order to obtain the maximum recovery rate.
Typical bar fed machines use reciprocating feed tubes to move the bar stock through the work spindles. Substantially all automatic screw machines, and some CNC lathes and other bar fed machines, use flexible feed fingers at the front end of the feed tube. The feed fingers grip the stock firmly enough so that they can move the stock when it is not clamped in place, but will slide over the bar stock when it is clamped. The feed fingers are used in connection with a chuck that grips the bar stock. To advance the stock, the feed fingers are retracted (moved away from the chuck), the chuck is opened, and the feed fingers are advanced (moved back toward the chuck), thereby advancing the bar stock through the chuck. The chuck then closes and regrips the bar stock to prepare for another cycle of machining operations.
At some point before the next time for advancing the bar stock, the feed fingers will again be retracted. The force that is necessary to retract the feed fingers will depend upon whether there is still bar stock in the fingers. If there is enough stock in the feed tube so that the feed fingers grip the bar stock for the entire retraction process, the force required to retract the feed tube will include the force required to drag the feed fingers over the bar stock, which is now clamped in the chuck. On the other hand, if the stock has been depleted and the feed fingers slip off the stock at any time in the retraction process, the force required to move the feed tube will just be the amount required to overcome friction in the feed tube slide assembly. This is typically on the order of 1 to 5 lbs, as opposed to about 8 to 150 lbs to retract the feed tube assembly when the feed fingers are gripping bar stock.
Attempts have been made to use the change in force required to move the feed tube as a means of sensing stock depletion. One such system relies upon the momentum of the feed slide assembly to carry the slide far enough, when stock is depleted, to operate a micro switch. Due to variations in the drag on the feed tube and feed slide, however, this method is not fully dependable.
Another form of stock depletion sensor for automatic screw machines, commonly called a short part detector, positions a sensor in front of a spindle where it can detect that not enough stock was fed out. However, when a bar is already fed out, that spindle is no longer at the machine station where stock can be easily removed or a new bar loaded. The resulting drawback of a short part detector is that a short piece of stock has now been introduced into the tooling area and there is no way to remove it. Depending on tooling, the operator may have to cut the short piece off with a hand saw and then index the machine around until the empty spindle returns to the bar load station. Conversely, in the case of the stock depletion system, the short remnant can be removed and a new bar can be loaded immediately either automatically by a bar loader, or manually by the operator.